The subject matter relates to a dual-crankshaft, opposed-piston engine equipped for variable crankshaft phasing in order to change port timing and/or port phasing in response to changing engine conditions. Particularly, the subject matter relates to an opposed-piston engine with two crankshafts coupled by a gear train in which a phasing mechanism coupled to at least one crankshaft varies port timing in the engine by changing the rotational phasing between the crankshafts, an operation referred to as “crank phasing”.
In an opposed-piston engine, a pair of pistons is disposed for opposed sliding motion in the bore of at least one ported cylinder. Each cylinder has exhaust and intake ports, and the cylinders are juxtaposed and oriented with exhaust and intake ports mutually aligned. Each port is constituted of one or more arrays or sequences of openings disposed circumferentially in the cylinder wall near a respective end of the cylinder. The engine includes two crankshafts rotatably mounted near respective exhaust ends and intake ends of the cylinders, and each piston is coupled to a respective one of the two crankshafts. The reciprocal movements of the pistons control the operations of the ports. In this regard, each port is located at a fixed position where it is opened and closed by a respective piston at predetermined times during each cycle of engine operation. Those pistons that control exhaust port operation are termed “exhaust pistons” and those that control intake port operation are called “intake pistons”.
Typically in opposed-piston engines the exhaust piston is phased in relation to the intake piston so as to enhance exhaust gas purging and scavenging during the later portion of the power stroke.
Piston phasing is normally fixed by positioning the exhaust piston connecting rod at some advanced angle on the crankshaft to which it is connected (“the exhaust crankshaft”) ahead of the intake piston connecting rod position on the crankshaft to which it is connected (“the intake crankshaft”). In such a configuration, as the pistons move away from top center (TC) positions after combustion, both ports (intake and exhaust) are closed by their respective pistons. As the pistons approach bottom center (BC) positions the exhaust port is opened first to begin exhaust gas purging and then the intake port opens some preset time later to allow pressurized air into the cylinder chamber to provide scavenging of the remaining exhaust gasses. Then, as the pistons reverse direction, the exhaust port closes first, allowing pressurized air into the cylinder chamber through the still open intake port until it too closes and a compression cycle begins.
It is desirable to be able to vary the timing or phasing of port openings and closings during engine operation in order to dynamically adapt the time that a port remains open to changing speeds and loads that occur during engine operation.
It is desirable to be able to vary the timing or phasing of port openings and closings during engine operation in order to maintain optimal blowdown, uniflow scavenging, and/or supercharger operations in the face of changing engine operating conditions.
Some opposed-piston engine designs do not utilize the pistons for port control. Instead, these engines are equipped with reciprocating sleeves that slide axially along the cylinder sidewall to open and close ports. Such arrangements are termed “sleeve valves” and port timing depends upon control of sleeve valve position and movement. Port phasing in sleeve valve engines presents very complicated control challenges that have to provide for timing the movements of crankshafts, pistons, and valve sleeves. Moreover, an important advantage of opposed-piston engines is the relative simplicity of engine construction: an opposed-piston engine dispenses with cylinder heads and many moving parts associated with valves and valve train mechanisms of single-piston engines. Much of this simplification is surrendered by the sleeve valve constructions.
It is therefore desirable to be able to control port phasing in an opposed-piston engine by relying on piston phasing to dynamically adapt port opening and closing times to changing speeds and loads that occur during engine operation. The objective is to secure the benefits realized by adapting port operation to varying engine operating conditions without sacrificing the simplifications achieved with opposed-piston constructions.